On most airlines, the standard formula for excess baggage is 1.5% of the applicable oneway fare per kilo... keep in mind that the applicable oneway fare can, and often will, be much higher than what you paid for your return...
For example, the cheapest LH
return from FRA
to NYC for 1 September is currently listed as €329 (excl. taxes), the cheapest oneway is €2432 - so the price per kilo would be €36.48! In other words, just 11 kilos of excess luggage would cost roughly the same as an extra ticket!
Admittedly, this example is a bit off - because LH
uses a different set of rules for the calculation of these charges... not sure how KLM handles this.
Excess baggage can be extremely expensive - find out how much more you'll be taking and call KLM and ask them how much the charge will be, so that you won't be too surprised at the airport.